Demodulation apparatus

ABSTRACT

A demodulation apparatus of the present invention comprises: an A/D converter for sampling and quantizing a baseband signal; a transversal filter having time-shifted tap coefficients; a decision unit for decoding the signal which has undergone the transversal filter; and a decision point estimation unit for instructing the transversal filter to select the tap coefficient to be selected based on information from the decision unit. Thus, the demodulation apparatus can operate at the same frequency as a sampling frequency of the A/D converter and can perform decision with accuracy equivalent to an arbitrary oversampling number.

FIELD OF THE INVENTION

[0001] The present invention relates to a demodulation apparatus for usein a digital transmission unit.

BACKGROUND OF THE INVENTION

[0002] Conventional demodulation of an input modulated wave in a digitaltransmission unit is performed in the following way. Ananalog-to-digital (A/D) converter samples an analog baseband signal at arate higher than a symbol rate (originally, a transmission rate ofdigital data), that is, a sampling frequency approximately sixteen timesas high as the symbol rate to convert it into a digital signal. Torestore the symbol rate, time-positions to be decided are estimated, andthe signal is then decimated. The signal further undergoes frequencyerror estimation, data decision and the others. It should be noted thatthere are cases where a digital filter, a phase converter or a delaydetector is provided at the subsequent stage of the A/D converter.

[0003] In the above case, the sampling frequency needs to be kept highfor the estimation of the time-positions to be decided, so that ademodulation apparatus needs to be operated at an increased frequency.Consequently, the demodulation apparatus can hardly be implemented in anintegrated circuit (IC) utilizing a generalized process or the like.

[0004] Moreover, with an increased occupied bandwidth of the signalreceived or the like, a frequency response and the others of the A/Dconverter are requested more strictly than ever.

[0005] There is one method known as a solution to the above problems.According to this method, an interpolator is provided at the subsequentstage of the A/D converter. Data undergo interpolation after samplingperformed by the A/D converter so as to be reproduced throughupsampling. The interpolation is done by inserting one data “0” or data“0” to the number of a given number into a string of data that isconverted into the digital signal by the A/D converter.

[0006] According to the above method, the sampling frequency can beequal to or higher than the Nyquist frequency covering a frequency bandof the input signal, thereby alleviating the requests to the A/Dconverter. However, the interpolation eventually requires a high-rateoperation, and the frequency at which the demodulation apparatus isoperated cannot thus be reduced. Accordingly, there is no choice but toincrease a clock frequency required for oversampling or upsampling, andthis circuit is thus hardly implemented in the IC utilizing thegeneralized process or the like.

SUMMARY OF THE INVENTION

[0007] The present invention addresses the problems discussed above andaims to reduce a frequency required for the operation of a demodulationapparatus.

[0008] The present invention utilizes redundancy of upsampling, so thata channel filter is comprised of a plurality of parallel transversalfilters. With this structure, even though a sampling rate required of anA/D converter is determined in consideration given to the effect ofinterference or the like that is present in the vicinity of a signalband, a clock frequency required of the transversal filters at thesubsequent stage can be equal to a symbol rate, and decision equivalentto an arbitrary oversampling number is feasible. Consequently, the clockfrequency of the demodulation apparatus can be reduced, the channelfilter having a flat group-delay response can be implemented, anddegradation of the demodulation apparatus can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram of a demodulation apparatus inaccordance with a first exemplary embodiment of the present invention.

[0010]FIG. 2 is a conceptual drawing of a data string for explaining theprinciple of the present invention.

[0011]FIG. 3 is a conceptual drawing of an impulse of a low pass filterfor explaining the principle of the present invention.

[0012]FIG. 4 is a conceptual drawing of a time relationship between thedata string and impulses for explaining the principle of the presentinvention.

[0013]FIG. 5 is a conceptual drawing for illustrating convolutionperformed in the present invention.

[0014]FIG. 6 is a block diagram of a demodulation apparatus inaccordance with a second exemplary embodiment of the present invention.

[0015]FIG. 7 is a block diagram of a demodulation apparatus inaccordance with a third exemplary embodiment of the present invention.

[0016]FIG. 8 is a block diagram of a demodulation apparatus inaccordance with a fourth exemplary embodiment of the present invention.

[0017]FIG. 9 is a block diagram of a demodulation apparatus inaccordance with a fifth exemplary embodiment of the present invention.

[0018]FIG. 10 is a block diagram of another demodulation apparatus inaccordance with the fifth exemplary embodiment of the present invention.

[0019]FIG. 11 is a block diagram of a demodulation apparatus inaccordance with a sixth exemplary embodiment of the present invention.

[0020]FIG. 12 is a waveform chart of impulse responses for explainingthe operation in accordance with the sixth exemplary embodiment of thepresent invention.

[0021]FIG. 13 is a block circuit diagram of a parallel-structuredtransversal filter of FIG. 11.

[0022]FIG. 14 is a block circuit diagram of a parallel-structureddecision point determination unit of FIG. 11.

[0023]FIG. 15 is a waveform chart illustrating the result of asimulation performed by the parallel-structured decision pointdetermination unit of FIG. 14.

[0024]FIG. 16 is a block circuit diagram of a maximum value designatingaddress detector of FIG. 14.

[0025]FIG. 17 is a block circuit diagram illustrating a transposedstructure of the parallel-structured transversal filter of FIG. 11.

[0026]FIG. 18 is a block circuit diagram illustrating the otherstructure of the parallel-structured transversal filter of FIG. 11.

[0027]FIG. 19 is a block circuit diagram illustrating another structureof the parallel-structured decision point determination unit of FIG. 11.

[0028]FIG. 20 is a block circuit diagram of a parallel-structuredtransversal filter in accordance with an eighth exemplary embodiment ofthe present invention.

[0029]FIG. 21 is a block diagram of a demodulation apparatus inaccordance with a ninth exemplary embodiment of the present invention.

[0030]FIG. 22 is a waveform chart of impulse responses for explainingthe operation in accordance with the ninth exemplary embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Exemplary embodiments of the present invention are demonstratedhereinafter with reference to the accompanying drawings.

Exemplary Embodiment 1

[0032]FIG. 1 is a block diagram in accordance with the first exemplaryembodiment of the present invention. Modulated wave 101 converted into abaseband undergoes sampling at a sampling frequency equal to or higherthan the Nyquist frequency covering a frequency band of wave 101 andquantisation in A/D converter 102 and is passed through transversalfilter 103 having time-shifted tap coefficients which designate decisionpoints on the time axis, respectively. An output of filter 103 undergoesfrequency error estimation, data decision and the others in decisionunit 104. Decision point estimation unit 105 estimates a decision pointbased on information output from decision unit 104 and instructs filter103 to select the tap coefficient designating the decision point.

[0033] An example of the use of interpolation done by insertion of “0”is explained hereinafter to facilitate the understanding of theprinciple of decision performed by transversal filter 103 of the presentinvention. It should be noted that the interpolation done by theinsertion of “0” is not actually performed in the present invention(described later).

[0034]FIG. 2 shows data “0” inserted into a string of data d0, d1, d2,d3, d4 . . . , which is converted into a digital signal by A/D converter102, for interpolation. Here, three data “0” are inserted into thestring of input data d0, d1, d2, d3, d4 . . . . From the viewpoint ofthe frequency, aliasing of the original sampling frequency is alsostored. To remove the aliasing, the signal input is low pass filtered bya low pass filter (LPF). In cases where the signal needs to undergo awaveform shaping filter, the waveform shaping can be carried outsimultaneously with the low pass filtering.

[0035] The low pass filtering is done by convolution of the data with animpulse of the LPF. FIG. 3 is a conceptual drawing of the impulse.

[0036] The convolution is performed on the data string after theinsertion of “0”. Its detail is explained with reference to FIG. 4 wherethe three “0”s are inserted (see FIG. 2). Assuming that n is anarbitrary integer and sampling data dn is in line with the center of animpulse at time 4n, data d2 is convolved with impulse p2 at time 4n.Similarly, at times 4n+1, 4n+2 and 4n+3, the “0”s inserted behind d2 insuccession are convolved with impulses p2+1, p2+2 and p2+3,respectively.

[0037] As is clear from the above explanation, since sampling data dnwhich undergoes the convolution is positioned only in a specified place,the impulse shifted in time with respect to data dn free of “0” ispresumably convolved at each time (see FIG. 5). Focusing attention onthis point, the present invention utilizes a time shift of the impulsewith respect to the data string for the convolution without inserting“0”. In other words, the impulse shifted in time with respect to data dnfree of “0” is utilized for the convolution at each time because, asshown in FIG. 5, sampling data dn which undergoes the convolution ispositioned only in a specified place, so that the insertion of “0” neednot be actually performed.

[0038] The convolution of the time-shifted impulses corresponds tosampling at a high sampling frequency, so that to restore the originalsampling frequency, that is, a symbol rate, the signal requiresdecimation. The decimation corresponds to selecting one impulse that isshifted in time. Therefore, if how much time shifts is detected for thedecimation, as shown in FIG. 5, an impulse time shifted by Δt isprepared beforehand and then convolved; this is equivalent to samplingat the original sampling frequency, so that the decimation isunnecessary.

[0039] Transversal filter 103 is a transversal filter having the tapcoefficients where the time-shifted impulses correspond to respectivetimings of the sampling data, respectively. Thus, the insertion of “0”that requires high-rate operation is unnecessary, the LPF can besimplified, and filter 103 can be operated at the same frequency as thatof A/D converter 102.

[0040] Each of the tap coefficients of filter 103 can be prepared andselected by a read only memory (ROM) according to the time shift ofrequired accuracy. Instead of the ROM, a digital signal processor (DSP)or the like can be used to determine which of the tap coefficientsshould be selected.

[0041] There is no limitation on the operation of decision pointestimation unit 105. Estimation unit 105 may estimate the decision pointthrough enveloped square-law detection or the use of a phase componentof a pilot signal.

Exemplary Embodiment 2

[0042]FIG. 6 is a block diagram in accordance with the second exemplaryembodiment of the present invention. Modulated wave 701 converted into abaseband undergoes sampling at a sampling frequency equal to or higherthan the Nyquist frequency covering a frequency band of wave 701 andquantisation in A/D converter 702 and is passed through transversalfilters 703, 704. Selector 705 selects one output from respectiveoutputs of filters 703, 704, and the output selected undergoes frequencyerror estimation, data decision and the others in decision unit 706.Decision point estimation unit 707 estimates a decision point based oninformation output from decision unit 706 and instructs filters 703, 704to respectively prepare tap coefficients each designating the decisionpoint, and selector 705 selects one effective output from respectiveoutputs of filters 703, 704.

[0043] Only a part of the operation of a demodulation apparatus that isdifferent from that of the first exemplary embodiment is explained next.There are cases where transversal filter 103 of FIG. 1 does not producethe output while switching to the tap coefficient desired and until itsresult is reflected. In cases where this is undesirable, as shown inFIG. 6, two transversal filters 703, 704 are prepared. While selector705 selects filter 703, filter 704 switches to a tap coefficientdesired, and upon completion of the processing performed by filter 703,selector 705 selects filter 704. The output thus comes from eitherfilter 703 or filter 704 at all times and is always guaranteed.

Exemplary Embodiment 3

[0044]FIG. 7 is a block diagram in accordance with the third exemplaryembodiment of the present invention. Modulated wave 801 converted into abaseband undergoes sampling at a sampling frequency equal to or higherthan the Nyquist frequency covering a frequency band of wave 801 andquantisation in A/D converter 802 and is passed through transversalfilters 803, 804, 805. Selector 806 selects one output from respectiveoutputs of filters 803, 804, 805, and the output selected undergoesfrequency error estimation, data decision and the others in decisionunit 807.

[0045] Decision point estimation unit 808 estimates a decision pointbased on information output from decision unit 807 and instructs filters803, 804, 805 to respectively prepare tap coefficients each designatingthe decision point, and selector 806 selects one effective output fromrespective outputs of filters 803, 804, 805.

[0046] Only a part of the operation of a demodulation apparatus that isdifferent from that of the second exemplary embodiment is explainednext. FIG. 6 is based on the assumption that the selection of the tapcoefficient is made according to the decision point arbitrarily set.However, it is common practice to make fine backward-and-forwardadjustments of the decision point in demodulation. Accordingly, tapcoefficients which are time shifted forward and backward with respect tothe time of the transversal filter selected are respectively prepared inthe other transversal filters which are not selected by selector 806.For example, in cases where transversal filter 803 is selected byselector 806, a tap coefficient time shifted slightly backward isprepared in transversal filter 804, while a tap coefficient time shiftedslightly forward is prepared in transversal filter 805. When selector806 selects filter 804 to shift the time slightly backward, the tapcoefficient of filter 803 is unchanged, so that it becomes as if itshifts forward with respect to filter 804 selected. On the other hand,filter 805 switches to a tap coefficient shifted backward with respectto filter 804 selected. The backward-and-forward time shift can thus beperformed smoothly at all times by changing the tap coefficient of oneof the transversal filters.

[0047] The width of the forward-and-backward time shift may be fixed.The forward-and-backward time shift may be widened when accuracy of thedecision point is poor and may be changed to shorten when accuracy ofthe decision point increases.

Exemplary Embodiment 4

[0048]FIG. 8 is a block diagram in accordance with the fourth exemplaryembodiment of the present invention. Modulated wave 901 converted into abaseband undergoes sampling at a sampling frequency equal to or higherthan the Nyquist frequency covering a frequency band of wave 901 andquantisation in A/D converter 902 and is passed through transversalfilters 903, 904, 905. Selector 906 selects one output from respectiveoutputs of filters 903, 904, 905, and the output selected undergoesfrequency error estimation, data decision and the others in decisionunit 907.

[0049] Decision point estimation unit 908 estimates a decision pointbased on the respective outputs of filters 903, 904, 905 and instructsfilters 903, 904, 905 to respectively prepare tap coefficients eachdesignating the decision point, and selector 906 selects one effectiveoutput from respective outputs of filters 903, 904, 905.

[0050] Only a part of the operation of a demodulation apparatusdifferent from that of the third exemplary embodiment is explained next.Decision point estimation unit 908 estimates the decision point byutilizing the respective outputs of filters 903, 904, 905 before theseoutputs undergo selector 906.

Exemplary Embodiment 5

[0051]FIG. 9 is a block diagram in accordance with the fifth embodimentof the present invention. Modulated wave 1001 having an intermediatefrequency band, the center frequency of which is a quarter of a samplingfrequency (described later), undergoes sampling at the samplingfrequency two or more times as high as the Nyquist frequency coveringthe frequency band of wave 1001 when wave 1001 is converted into abaseband and quantisation in A/D converter 1002 and is split into anin-phase (I) signal and a quadrature (Q) signal. The signals aremultiplied by respective signals from digital local oscillators 1009,1010, respectively in multipliers 1003, 1004, respectively. Respectiveoutputs of multipliers 1003, 1004 are passed through transversal filters1005, 1006, respectively and undergo frequency error estimation, datadecision and the others in decision unit 1007. Decision point estimationunit 1008 estimates a decision point based on information output fromdecision unit 1007 and instructs filters 1005, 1006 to respectivelyprepare tap coefficients each designating the decision point.

[0052] The signals of digital local oscillators 1009, 1010 becomesimplest factors when the center frequency is a quarter of the samplingfrequency as mentioned above and are a repetition of 1, 0, −1, 0 withrespect to the I signal and a repetition of 0, −1, 0, 1 with respect tothe Q signal. In the case of other frequencies, a digital directsynthesizer or the like is applied to the local oscillator.

[0053] The signals input to transversal filters 1005, 1006 are of thesampling frequency two or more times as high as the Nyquist frequency.

[0054] As shown in FIG. 10, this sampling frequency can be reduced tothe half by the known method. Its detail is omitted since it is publiclyknown and disclosed in a data sheet on HSP43216 of Intersil Corporationand the others. Its summary is as follows.

[0055] In FIG. 10, modulated wave 1101 having an intermediate frequencyband undergoes sampling and quantisation in A/D converter 1102 andundergoes distributor 1103 to be alternately split into an in-phase (I)signal and a quadrature (Q) signal. The signals are multiplied byrespective outputs of digital local oscillators 1112, 1113, respectivelyin multipliers 1104, 1105, the respective outputs of which undergohalf-band filter 1106 comprising only even taps on the I signal side andhalf-band filter 1107 comprising only odd taps on the Q signal side,respectively, so that the speed of data is reduced to the half.Thereafter, the data undergo transversal filters 1108, 1109 and thenundergo frequency error estimation, data decision and the others indecision unit 1110.

[0056] Decision point estimation unit 1111 estimates a decision pointbased on information output from decision unit 1110 and instructsfilters 1108, 1109 to respectively prepare tap coefficients eachdesignating the decision point. The outputs of local oscillators 1112,1113 are a repetition of 1, −1 with respect to the I signal and arepetition of −1, 1 with respect to the Q signal. The combination offilter 1106 (1107) and filter 1108 (1109) is seemingly redundant, but afrequency at which a demodulation apparatus operates is halved.

Exemplary Embodiment 6

[0057]FIG. 11 is a block diagram of a demodulation apparatus inaccordance with the sixth exemplary embodiment of the present invention.The apparatus is comprised of A/D converter 1501, distributor 1502,parallel-structured transversal filter 1503, parallel-structureddecision point determination unit 1504, selector 1505 and decision unit1506.

[0058] A signal A/D converted by converter 1501 is split into aplurality of signals of the same kind by distributor 1502 and then inputto a plurality of input terminals of transversal filter 1503,respectively. In consideration given to a fan-out, distributor 1502requires a flip-flop or the like so that it can sufficiently drive theinput of all of the signals to transversal filter 1503.

[0059] The following is an explanation of parallel-structuredtransversal filter 1503. To achieve impulse responses shown in FIG. 12,transversal filter 1503 is a parallel combination of structures eachhaving tap coefficients, like those shown in FIG. 13. In the presentembodiment illustrated by FIGS. 12 and 13, the number of structures inparallel is four, and filter 1503 performs decision on the signal, whichis sampled by A/D converter 1501 at a rate twice as high as a symbolrate, with accuracy equivalent to eight-times oversampling. Each of thefour structures of transversal filter 1503 is comprised of: delaycircuits 1701-1705 in series; weighting circuits 1706-1711 forextracting signals from connection midpoints of circuits 1071-1705 andamplitude-regulating the signals; and adding circuit 1712 for addingrespective outputs of weighting circuits 1706-1711. An output of addingcircuit 1712 becomes an output signal of transversal filter 1503.

[0060] The change of the number of structures of the transversal filter1503 allows transversal filter 1503 to perform decision with accuracyequivalent to 2×N-times oversampling (where N is a natural numberrepresenting the number of structures in parallel).

[0061] Here, the assumption is made that a channel filter on atransmitting side restricts a signal to a frequency band two or lesstimes as high as the symbol rate. The A/D conversion at the samplingrate twice the symbol rate can thus satisfy the Nyquist theorem as wellas the theory of upsampling. Through the use of, for example, a routeroll-off filter as the channel filter for restricting the signal to thefrequency band two or less times as high as the symbol rate, when aroll-off factor is 1.0, even the signal having a maximum frequency bandis restricted to the frequency band twice the symbol rate, whichsituation is preferable. Transversal filter 1503, a channel filter on areceiving side, is also a route roll-off filter, and since its impulseresponses are evenly symmetric with a maximum value being at its center,a group-delay response of transversal filter 1503 having these impulseresponses sampled as the tap coefficients is flat, which situation ispreferable.

[0062] Based on a plurality of signals simultaneously output fromparallel-structured transversal filter 1503, an optimum decision pointis determined. This is done by parallel-structured decision pointdetermination unit 1504. The output signal that corresponds to a phaseof the decision point determined is selected by selector 1505 from theoutput signals of transversal filter 1503 and then input to decisionunit 1506.

[0063] Decision point determination unit 1504 can have a structure, likethe one shown in FIG. 14. Specifically, it is comprised ofdemultiplexers 1081, squaring units 1802, factor β multipliers 1803,adders 1804, factor 1-β multipliers 1805, delay circuits 1806 eachdelaying an output passing through circuit 1806 by a symbol cycle time,maximum value designating address detector 1807 and the others.Multipliers 1803, adders 1804, multipliers 1805, delay circuits 1806 anddetector 1807 form a low pass filter. Each of the output signals oftransversal filter 1503 is split by demultiplexer 1081. Each of theresults split is squared by squaring unit 1802, multiplied by factor βand added to a value obtained by multiplying a cumulative value of delaycircuit 1806 by factor 1-β. The result added accumulates in delaycircuit 1806.

[0064] As shown in FIG. 14, the number of structures—each being formedby demultiplexer 1801, two squaring units 1802, two multipliers 1803,two adders 1804, two multipliers 1805 and two delay circuits 1806—isprepared in parallel so as to be equal to the number of structures ofparallel-structured transversal filter 1503. Among cumulative values ofthese structures that have accumulated to some symbols, maximum valuedesignating address detector 1807 detects the cumulative value that ismaximum. The thus detected output signal of transversal filter 1503 isdesignated as the optimum decision point.

[0065]FIG. 15 shows, as an example, the result of simulated values atrespective phases that are input to detector 1807 when the values haveaccumulated to a hundred symbols, along with an eye diagram. In thiscase, roll-off factor α=0.5, factor β=0.01, and a random signal is inputto the demodulation apparatus.

[0066] As shown in FIG. 15, a maximum value designates the optimumdecision point (the phase with a maximum eye aperture ratio) not onlywhen a carrier-to-noise ratio (C/N) is infinite but also when C/N=10 dB.

[0067] After maximum value designating address detector 1807 designatesthe maximum value as the decision point (address), selector 1505 selectsthe output signal of transversal filter 1503 that corresponds to thedecision point designated, and the output signal selected undergoesdecision unit 1506. Selector 1505 makes a selection by determining whichof the output signals of transversal filter 1503 designates the addressdetected by detector 1807.

[0068] In other words, selector 1505 makes a selection by determiningwhich of demultiplexers 1801 has the input signal corresponding to theaddress. Specifically, selector 1505 uses only two higher-order bits ofthe detected address for the selection in cases where the decision pointhaving an effect equivalent to eight-times oversampling is desired.Since the signal of a rate twice the symbol rate is input from selector1505 to decision unit 1506, decision unit 1506 makes a decision on thesignal every second time.

[0069] As shown in FIG. 14, the decision point selected by detector 1807is also input to decision unit 1506. This favorably facilitates decisionas to which of the two times is a time suitable for the decision.Specifically, only the lowest-order bit of the address detected is usedto select the time for the decision.

[0070] Maximum value designating address detector 1807 can detect theaddress, that is, the maximum value at the same clock rate as the priorstage and determine which of the output signals of transversal filter1503 designates the optimum decision point, provided that it tolerateslatency equivalent to some clocks. Detector 1807 includes a combinationof a plurality of comparators shown in FIG. 16 to tolerate latencyequivalent to three clocks. Detector 1807 can thus detect the decisionpoint with accuracy equivalent to eight-times oversampling, while A/Dconverter 1501 and transversal filter 1503 at the subsequent stage ofA/D converter 1501 are operated at the rate twice the symbol rate.

[0071] Other than the structure shown in FIG. 13, it is also preferablethat parallel-structured transversal filter 1503 has, as shown in FIG.17, a transposed structure in which weighting circuits 2101-2106 areconnected in parallel, and respective outputs of weighting circuits2101-2105 are delayed by delay circuits 2107-2111 and then accumulate inadding circuits 2112-2116, respectively.

Exemplary Embodiment 7

[0072] The seventh exemplary embodiment has the same block diagram (seeFIG. 11) as that of the sixth exemplary embodiment. However, a clockrate required of parallel-structured transversal filter 1503, selector1505 and parallel-structured decision point determination unit 1504 thatare placed at the subsequent stage of A/D converter 1501 is the same asthe symbol rate for the purpose of placing demultiplexers at thesubsequent stage of A/D converter 1501 thereby to reduce the operationspeed of transversal filter 1503 in half. The detailed structure isshown FIG. 18.

[0073]FIG. 18 shows transversal filter 1503 that performs decision ofdecision points from a signal, which is sampled by A/D converter 2201 ata rate twice as high as a symbol rate, with accuracy equivalent toeight-times oversampling for the purpose of achieving impulse responses,like those of FIG. 12.

[0074] The baseband signal converted by A/D converter 2201 is split intotwo, with one being delayed by an A/D conversion sampling time (half asymbol cycle time) in delay circuit 2202, split into four outputs andfed to demultiplexers 2204, 2206, 2208, 2210, respectively and the otherbeing split into four outputs without being delayed and fed todemultiplexers 2203, 2205, 2207, 2209, respectively.

[0075] After demultiplexers 2203, 2205, 2207, 2209 split the fouroutputs into two outputs, respectively, the resulting outputs are fed totransversal filters 2215, 2216, 2219, 2220, 2223, 2224, 2227, 2228,respectively. Respective outputs of transversal filters 2215, 2216 areadded together by adder 2231. Similarly, respective outputs oftransversal filters 2219, 2220, respective outputs of transversalfilters 2223, 2224 and respective outputs of transversal filters 2227,2228 are added together by adders 2233, 2235, 2237, respectively.

[0076] The other demultiplexers 2204, 2206, 2208, 2210 split the fouroutputs into two outputs, respectively, and some of the resultingoutputs are fed to transversal filters 2217, 2221, 2225, 2229, while theother resulting outputs are delayed by a time twice the A/D conversionsampling time, that is, the symbol cycle time in delay circuits 2211,2212, 2213, 2214 and fed to transversal filters 2218, 2222, 2226, 2230,respectively. Respective outputs of transversal filters 2217, 2218 areadded together by adder 2232. Similarly, respective outputs oftransversal filters 2221, 2222, respective outputs of transversalfilters 2225, 2226 and respective outputs of transversal filters 2229,2230 are added together by adders 2234, 2236, 2238, respectively.

[0077] Respective outputs of adders 2231-2238 are input to selector1505, like the one shown in FIG. 11, and parallel-structured decisionpoint determination unit 2301 shown in FIG. 19.

[0078] The number of inputs to selector 1505 and determination unit 2301needs to be doubled compared with that of the case of FIG. 11, while thespeed at which selector 1505 and determination unit 2301 operate may behalf the operation speed of the case shown in FIG. 11.Parallel-structured decision point determination unit 2301 of FIG. 19has the same structure as that of FIG. 14 except that it has nodemultiplexer 1801. Decision unit 1506 placed at the subsequent stage ofselector 1505 differs from that of the sixth embodiment in that a signalinput thereto is of a rate coinciding with the symbol rate, so thatdecision unit 1506 needs to make a decision every time.

[0079] To achieve the impulse responses, like those shown in FIG. 12,parallel-structured transversal filter 1503 comprises: filters 2215,2217 each having a group of tap coefficients c0, c8, c16; filters 2216,2218 each having a group of tap coefficients c4, c12, c20; filters 2219,2221 each having a group of tap coefficients c1, c9, c17; filters 2220,2222 each having a group of tap coefficients c5, c13, c21; filters 2223,2225 each having a group of tap coefficients c2, c10, c18; filters 2224,2226 each having a group of tap coefficients c6, c14, c22; filters 2227,2229 each having a group of tap coefficients c3, c11, c19; and filters2228, 2230 each having a group of tap coefficients c7, c15, c23. Inother words, the tap coefficients in each of the coefficient strings oftransversal filter 1503 in accordance with the sixth embodiment arealternately included in separate filters, so that the speed required forthe multiplication is halved, and through the addition of the resultsmultiplied, the same response as that of the sixth embodiment can beobtained.

[0080] In the present embodiment, only A/D converter 2201 performssampling at the rate twice the symbol rate, and subsequentdemultiplexers 2203-2210 operate at the clock rate equal to the symbolrate.

[0081] It is also preferable that parallel-structured transversal filter1503 constructed in accordance with the present embodiment has atransposed structure, like the one shown in FIG. 17, in place of thestructure shown in FIG. 13.

Exemplary Embodiment 8

[0082] The eighth exemplary embodiment utilizes the same principle asthat of the seventh exemplary embodiment, and is a case where a samplingrate of an A/D converter is N times as high as a symbol rate where N isa natural number equal to or greater than 2. FIG. 20 shows one exampleof a structure achieving impulse responses, like those of FIG. 12, whenN=4.

[0083] The structure illustrated by FIG. 20 is comprised of: A/Dconverter 2401, the sampling rate of which is four times as high as thesymbol rate; delay circuits 2402-2407; demultiplexers 2408-2415;transversal filters 2416 each having a group of tap coefficients c0, c8,c16; transversal filters 2417 each having a group of tap coefficientsc1, c9, c17; transversal filters 2418 each having a group of tapcoefficients c2, c0, c18; transversal filters 2419 each having a groupof tap coefficients c3, c11, c19; transversal filters 2420 each having agroup of tap coefficients c4, c12, c20; transversal filters 2421 eachhaving a group of tap coefficients c5, c13, c21; transversal filters2422 each having a group of tap coefficients c6, c14, c22; transversalfilters 2423 each having a group of tap coefficients c7, c15, c23; andadders 2424, 2425. Demultiplexers 2408-2415 demultiplexes one signalinto four signals.

[0084] A baseband signal A/D converted by converter 2401 is split intoeight outputs. Some of the outputs are delayed by an A/D conversionsampling time (half a symbol cycle time) in delay circuits 2402-2407 andfed to demultiplexers 2409-2411 and 2413-2415, respectively.Demultiplexers 2409-2411 split respective outputs of delay circuits2402-2404 into four outputs, respectively, and the four outputs of eachof demultiplexers 2409-2411 are fed to transversal filters 2416, 2418,2420, 2422, respectively. Demultiplexers 2413-2415 split respectiveoutputs of delay circuits 2405-2407 into four outputs, respectively, andthe four outputs of each of demultiplexers 2413-2415 are fed totransversal filters 2417, 2419, 2421, 2423, respectively. Respectiveoutputs of transversal filters 2416, 2418, 2420, 2422 are added togetherby adder 2424, while respective outputs of transversal filters 2417,2419, 2421, 2423 are added together by adder 2425.

[0085] The other outputs of A/D converter 2401 that do not undergo delaycircuits 2402-2407 are fed to demultiplexers 2408, 2412, respectively.Demultiplexer 2408 splits the output into four outputs, which are fed totransversal filters 2416, 2418, 2420, 2422, respectively and addedtogether by adder 2424, while demultiplexer 2412 splits the output intofour outputs, which are fed to transversal filters 2417, 2419, 2421,2423, respectively and added together by adder 2425.

[0086] Through the preparation of the respective tap coefficient groupsof transversal filters 2416-2423 in the same manner as the seventhembodiment and the parallel arrangement of a plurality of filters2416-2423 each having phase-shifted tap coefficients, a demodulationapparatus capable of performing decision with accuracy equivalent to anarbitrary oversampling number can be realized, with only the samplingrate of A/D converter 2401 being high, while processing of the signal atthe subsequent stage thereof being carried out at the symbol rate.

[0087] As described above, the demodulation apparatus for the digitalmodulated wave according to the present invention can performdemodulation equivalent to the arbitrary oversampling number whilekeeping the increase of the clock frequency, which is required whenreceiving a broadband signal, to a bare minimum and delivers highperformance even when an interference signal is present in the vicinityof the broadband signal.

Exemplary Embodiment 9

[0088]FIG. 21 shows exemplary embodiment 9 of the present invention. Thedemodulation apparatus essentially comprises an AD converter 2501,filter coefficient memories 2502, 2503 each of which stores filtercoefficients for designating tap coefficients of transversal filters2504, 2505 respectively, demultiplexers 2506,2507, square circuits2508-2511 connected to output terminals of the demultiplexers 2506,2507,low pass filters 2512-2515 each of which is connected to outputs of thesquare circuits 2508-2511,a selector 2516 connected to the outputs ofthe demultiplexers 2506,2507, a filter coefficient determining unit2517connected to outputs of the low pass filters 2512-2515, a decisionpoint determining unit 2518 connected to other outputs of the low passfilters 2512-2515, and a decision unit 2519.

[0089] A baseband signal inputted to the AD converter 2501 is convertedinto digital signal and divided into two signals. One of which isinputted to a transversal filter 2504 and another is inputted to atransversal filter 2505. A filter coefficient of the transversal filter2504 is settled to a sampling values for designating sampling points{a0, j0, a1, j1, . . . } of FIG. 22 as a initial condition, while afilter coefficient of the transversal filter 2505 is settled to asampling values for designating {e0, m0, e1, m1, . . . } of FIG. 22 as ainitial condition. The sampling points {e0, m0, e1, m1, . . . }correspond to points by shifting phases of the {a0, j0, a1, j1, . . . }with 90 degrees. Output signals of the transversal filters 2504, 2505are demultiplexed by the demultiplexers 2506, 2507 and supplied to theselector 2516. The output signals of the demultiplexers 2506, 2507 arealso supplied to the square circuits 2508-2511. Each of the outputs ofthe square circuits 2508-2511 are supplied to the low pass filters2512-2515, respectively. Each of the low pass filter 2512-2515 isdesigned to cumulate inputted data thereto during a predetermined timeand reset them after elapsed the predetermined time.

[0090] The cumulated output signal of each of the low pass filters2512-2515 is supplied to the filter coefficient determining unit 2517and the decision point determining unit 2518 respectively. The decisionpoint determining unit 2518 detects an address showing a maximum valueof four output signals of the low pass filters 2512-2515, and theaddress is noticed to the selector 2516. The selector selects one outputsignal, which is obtained from the signal pass having the addressnoticed by the decision point determining unit 2518, of the outputsignals of the demultiplexers 2506, 2507. The selected signal issupplied to the decision unit 2519 and decides data contents such asdata position on I-Q co-ordinates.

[0091] The filter coefficient determining unit 2517 also detects theaddress before resetting the low pass filter 2512-2515, and givesinformation to the filter coefficient memories 2502,2503 to select a tapcoefficient group which corresponds to points by shifting phase with ±45degrees from the phase corresponding to the detected address. Forexample, if the output corresponding to phase e of FIG. 22 is a maximumvalue, the tap coefficients group {c0, k0, c1, k1, . . . }, which areshifted with −45 degrees from the phase e0, and {g0, o0, g1, o1, . . .}, which are shifted with +45 degrees from the phase e0, are selected.

[0092] In second stage, the filter coefficient determining unit 2517detects second stage address showing second stage maximum value of fouroutput signals of the low pass filters 2512-2515 before resetting thelow pass filter 2512-2515, and gives second stage information to thefilter coefficient memories 2502,2503 to select a tap coefficient groupwhich corresponds to points by shifting phase with ±45/2 degrees fromthe phase corresponding to the second stage address. For example, if theoutput corresponding to phase g is a maximum value, the tap coefficientsgroup {f0, n0, f1, n1, . . . } and {h0, p0, h1, p1, . . . } areselected.

[0093] In third stage, the filter coefficient determining unit 2517detects third stage address showing third stage maximum value of fouroutput signals of the low pass filters 2512-2515 before resetting thelow pass filter 2512-2515, and gives third stage information to thefilter coefficient memories 2502,2503 to select a tap coefficient groupwhich corresponds to points by shifting phase with +45/4 degrees or−45/4 degrees from the phase corresponding to the third stage address.The selection of +45/4 degrees or −45/4 degrees is performed in sameprobability. For example, if the output corresponding to phase f is amaximum value, the tap coefficients group {f0, n0, f1, n1, . . . } and{e0, m0, e1, m1, . . . } are selected. In next stage, if the outputcorresponding to phase f is a maximum value again, the tap coefficientsgroup {f0, n0, f1, n1, . . . } and {g0, o0, g1, o1, . . . } areselected.

What is claimed is:
 1. A demodulation apparatus comprising: an A/Dconverter for sampling and quantizing a baseband signal; a firsttransversal filter, having tap coefficients to the number of a givennumber that are shifted in time with respect to each other, forreceiving an output signal from the A/D converter; a decision unit formaking a decision on an output signal of the first transversal filter;and a decision point determination unit for instructing the firsttransversal filter to select the tap coefficient to be selected based onan output signal of the decision unit.
 2. The demodulation apparatus ofclaim 1, further comprising: a branch unit for splitting the outputsignal of the A/D converter into two output signals; a secondtransversal filter for receiving one of the output signals from thebranch unit; and a selector for selecting one output signal fromrespective output signals of the first and second transversal filters,wherein the first transversal filter receives the other output signalfrom the branch unit in place of the output signal from the A/Dconverter, and wherein the decision point determination unit instructsthe selector to select the transversal filter to be selected based onthe output signal of the decision unit.
 3. The demodulation apparatus ofclaim 1, further comprising: a branch unit for splitting the outputsignal of the A/D converter into a first output signal, a second outputsignal and a third output signal; a second transversal filter forreceiving the second output signal from the branch unit; a thirdtransversal filter for receiving the third output signal from the branchunit; and a selector for selecting one output signal from respectiveoutput signals of the first, second and third transversal filters,wherein the first transversal filter receives the first output signalfrom the branch unit in place of the output signal from the A/Dconverter, and wherein the decision point determination unit instructsthe first, second and third transversal filters to respectively preparetap coefficients that are shifted in time at equal intervals withrespect to each other and instructs the selector to select thetransversal filter to be selected based on the output signal of thedecision unit.
 4. The demodulation apparatus of claim 3, wherein thedecision point determination unit instructs the first, second and thirdtransversal filters to respectively prepare the tap coefficients basedon the respective output signals of the transversal filters in place ofthe output signal of the decision unit.
 5. A demodulation apparatuscomprising: an A/D converter for sampling and quantizing an inputsignal, that is, an analog signal of an intermediate frequency; amultiplier for frequency-converting an output of the A/D converter; atransversal filter, having tap coefficients to the number of a givennumber that are shifted in time with respect to each other, forreceiving an output signal of the multiplier; a decision unit for makinga decision on an output signal of the transversal filter; and a decisionpoint determination unit for instructing the transversal filter toselect the tap coefficient to be selected based on an output signal ofthe decision unit.
 6. A demodulation apparatus comprising: an A/Dconverter for sampling and quantizing an analog input signal of anintermediate frequency at a sampling frequency four times as high as acenter frequency of the analog signal; a distributor for splitting thesignal A/D converted into an in-phase signal and a quadrature signal; afirst multiplier for multiplying the in-phase signal by a signal from afirst local oscillator; a second multiplier for multiplying thequadrature signal by a signal from a second local oscillator; a firsttransversal filter through which an output of the first multiplier ispassed; a second transversal filter through which an output of thesecond multiplier is passed; a decision unit for decoding respectiveoutput signals of the first and second transversal filters; and adecision point determination unit for instructing the first and secondtransversal filters to respectively prepare tap coefficients based on apiece of information of the decision unit.
 7. The demodulation apparatusof claim 6, wherein the sampling frequency is two or more times as highas the Nyquist frequency.
 8. The demodulation apparatus of claim 6,wherein the signal of the first local oscillator is a repetition of 1,0, −1, 0, while the signal of the second local oscillator is arepetition of 0, −1, 0,
 1. 9. The demodulation apparatus of claim 6,further comprising: a first half-band filter, having even taps, forfiltering the output of the first multiplier; and a second half-bandfilter, having odd taps, for filtering the output of the secondmultiplier, wherein an output of the first half-band filter is input tothe first transversal filter, while an output of the second half-bandfilter is input to the second transversal filter.
 10. The demodulationapparatus of claim 9, wherein the signal of the first local oscillatoris a repetition of 1, −1, 1, −1, while the signal of the second localoscillator is a repetition of −1, 1, −1,
 1. 11. A demodulation apparatuscomprising: an A/D converter for sampling and quantizing an analogbaseband signal; a branch unit for splitting an output of the A/Dconverter into L outputs where L is an integer equal to or greater than2; a parallel-structured transversal filter having L transversal filtersin parallel for receiving the outputs from the branch unit,respectively; a parallel-structured decision point determination unitfor selecting one output signal from respective output signals of the Ltransversal filters of the parallel-structured transversal filter; aselector for outputting only the output signal of the transversal filterthat is selected by the parallel-structured decision point determinationunit to a subsequent stage thereof; and a decision unit for making adecision on the output signal of the selector.
 12. The demodulationapparatus of claim 11, wherein each of the transversal filters of theparallel-structured transversal filter comprises: a plurality of delaycircuits placed in series; weighting circuits for extracting signalsfrom connection midpoints of the delay circuits and amplitude-regulatingthe signals, respectively; and an adding circuit for adding respectiveoutputs of the weighting circuits.
 13. The demodulation apparatus ofclaim 11, wherein each of the transversal filters of theparallel-structured transversal filter comprises: a plurality ofweighting circuits placed in parallel; delay circuits for delayingrespective outputs of the weighting circuits, respectively; and addingcircuits, each of which adds the output of the weighting circuit to anoutput of each of the delay circuits.
 14. The demodulation apparatus ofclaim 11, wherein the parallel-structured decision point determinationunit comprises a plurality of structures placed in parallel, whereineach of the structures comprises: a demultiplexer for demultiplexing theoutput signal of each of the transversal filters of theparallel-structured transversal filter; squaring units for squaringrespective demultiplexed outputs, respectively; first factor multipliersfor multiplying respective outputs of the squaring units by a firstfactor, respectively; delay units for delaying outputs, whichrespectively pass through the delay units, by a symbol cycle time,respectively; second factor multipliers for multiplying respectiveoutputs of the delay units by a second factor, respectively; and adders,each of which adds an output of each of the first factor multipliers toan output of each of the second factor multipliers, wherein the parallelstructured decision point determination unit further comprises a maximumvalue designating address detector for detecting a maximum cumulativevalue among respective outputs of the structures.
 15. A demodulationapparatus comprising: an A/D converter for sampling and quantizing abaseband signal; a first branch unit for splitting an output of the A/Dconverter into two outputs; a first delay unit for delaying one of theoutputs of the first branch unit; a second branch unit for splitting theother output of the first branch unit into M outputs where M is aninteger equal to or greater than 2; a third branch unit for splitting anoutput of the first delay unit into M outputs; first demultiplexers tothe number of M, each of which demultiplexes each of the outputs of thesecond branch unit into two outputs; second demultiplexers to the numberof M, each of which demultiplexes each of the outputs of the thirdbranch unit into two outputs; second delay units, each of which delaysone of the outputs of each of the second demultiplexers; aparallel-structured transversal filter having 4M transversal filters inparallel for receiving the respective outputs of the firstdemultiplexers, the other outputs of the second demultiplexers andrespective outputs of the second delay units, respectively; 2M adders,each of which adds output signals of the parallel-structured transversalfilter on a demultiplexer-by-demultiplexer basis; a parallel-structureddecision point determination unit for selecting one output fromrespective outputs of the 2M adders; a selector for outputting only theoutput of the adder that is selected by the parallel-structured decisionpoint determination unit to a subsequent stage thereof; and a decisionunit for making a decision on the output of the selector.
 16. Thedemodulation apparatus of claim 15, wherein each of the transversalfilters of the parallel-structured transversal filter comprises: aplurality of delay circuits placed in series; weighting circuits forextracting signals from connection midpoints of the delay circuits andamplitude-regulating the signals, respectively; and an adding circuitfor adding respective outputs of the weighting circuits.
 17. Thedemodulation apparatus of claim 15, wherein each of the transversalfilters of the parallel-structured transversal filter comprises: aplurality of weighting circuits placed in parallel; delay circuits fordelaying respective outputs of the weighting circuits, respectively; andadding circuits, each of which adds the output of the weighting circuitto an output of each of the delay circuits.
 18. The demodulationapparatus of claim 15, wherein the parallel-structured decision pointdetermination unit comprises a plurality of structures placed inparallel, wherein each of the structures comprises: a squaring unit forsquaring the output signal of each of the transversal filters of theparallel-structured transversal filter; a first factor multiplier formultiplying an output of the squaring unit by a first factor; a delaycircuit for delaying an output passing through the delay circuit by asymbol cycle time; a second factor multiplier for multiplying an outputof the delay circuit by a second factor; and an adder for adding anoutput of the first factor multiplier to an output of the second factormultiplier, wherein the parallel-structured decision point determinationunit further comprises a maximum value designating address detector fordetecting a maximum cumulative value among respective outputs of thestructures.
 19. A demodulation apparatus comprising: an A/D converterfor sampling and quantizing a baseband signal at a sampling rate N-timesas high as a symbol rate where N is an integer equal to or greater than2; a first branch unit for splitting an output of the A/D converter intoP outputs where P is an integer equal to or greater than 2; a secondbranch unit for splitting each of the outputs of the first branch unitinto N outputs; P(N−1) delay units for delaying P(N−1) outputs of theoutputs of the second branch unit; PN demultiplexers for demultiplexingthe outputs, including respective outputs of the delay units and theoutputs of the second branch unit that are not delayed, into N signals,respectively; a parallel-structured transversal filter having PN²transversal filters in parallel for receiving the respective signals ofthe demultiplexers, respectively; PN adders, each of which adds outputsof the parallel-structured transversal filter on ademultiplexer-by-demultiplexer basis; a parallel-structured decisionpoint determination unit for selecting one output from respectiveoutputs of the PN adders; a selector for outputting only the output ofthe adder that is selected by the parallel-structured decision pointdetermination unit to a subsequent stage thereof; and a decision unitfor making a decision on the output of the selector.
 20. A demodulationapparatus comprising: an A/D converter for sampling and quantizing abaseband signal; a branch unit for splitting an output of the A/Dconverter into a plurality of output signals; a plurality of transversalfilters for receiving the respective output signals of the branch unitrespectively; a plurality of demultiplexers for demultiplexing theoutputs of the transversal filters respectively; a plurality of squareunits for operating square values of the outputs of the demultiplexersrespectively; a plurality of low pass filters for accumulating outputsignals of the square units during a predetermined period respectively;a decision point determination unit for detecting an address showing amaximum value of the output signals of the low pass filters andselecting one output from respective outputs of the demultiplexerscorresponding to the detected address; a filter coefficient determiningunit for detecting an address showing a maximum value of the outputsignals of the low pass filters during the every predetermined periodand determining filter coefficient of the transversal filters withphases determined by a phase corresponding to the detected address; aselector for outputting only the output of the demultiplexer that isselected by the decision point determination unit to a subsequent stagethereof; and a decision unit for making a decision on the output of theselector.
 21. The demodulation apparatus of claim 20, wherein the phasesare determined by shifting ±45 degrees from the phase corresponding tothe detected address in first process, by shifting ±45/2 degrees fromthe phase in second process, and by selecting one of ±45/4 degreesshifted phases from the phase after the second process.